Multifunctional poly(vinyl alcohol) binder for fine particle size calcium carbonate pigment

ABSTRACT

Production of a paper coating composition having improved low shear viscosity at a high solids level of fine particle size calcium carbonate. The improvement in low shear viscosity is achieved by dissolving a fine particle size, partially hydrolyzed, low molecular weight poly(vinyl alcohol) powder in an aqueous slurry of pigment particles containing predominantly fine particle size calcium carbonate. Dissolution of the poly(vinyl alcohol) is achieved without external heating or adding water to the slurry. The slurry can then be formulated with other components to produce the ink jet paper coating composition which, when applied to a paper substrate, provides excellent ink jet printability.

BACKGROUND OF THE INVENTION

Paper coating compositions are used by the paper industry to impart thedesired strength and cosmetic properties to finished paper. The coatingcomposition is typically an aqueous dispersion consisting mainly ofmineral pigments, such as clay, calcium carbonate, silica, and titaniumdioxide, and pigment binders, such as starch and synthetic polymeremulsions. Coating compositions may also contain low levels ofadditives, such as thickeners, humectants and lubricants.

The coating compositions are usually applied to a continuous web ofcellulosic material, such as paper, by high speed coating machines, suchas blade coaters, air knife coaters, rod coaters and roll coaters. Thereare trends to use faster coaters to increase productivity and to usehigher solids coating compositions to decrease drying costs and improvebinder distribution which enhances paper quality.

Coatings which contain fine particle size pigments, such as calciumcarbonate, have been shown to be particularly useful in improving theproperties of ink jet recording paper. U.S. Pat. No. 5,643,631 (Donigianet al., 1997) and U.S. Pat. No. 5,783,038 (Donigian, et al., 1998)disclose thermal ink jet recording paper, incorporating heat agedprecipitated calcium carbonate and a binder, such as poly(vinylalcohol), starches, and carboxymethyl cellulose. Treatment of paper witha coating composition of a slurry of fine particle size calciumcarbonate in a poly(vinyl alcohol) or starch solution resulted inimproved optical density of ink jet print. An example of an appropriatepoly(vinyl alcohol) binder. was Airvol® 107 poly(vinyl alcohol) which is98 to 98.8% hydrolyzed. The binders were “cooked” to obtain a solutionprior to addition of the pigment slurry.

The use of poly(vinyl alcohol) and its derivatives as binders in ink jetcoating systems are well known in the art. For example, an article by C.A. Finch in Polyvinyl Alcohol—Developments, Wiley, 1992, pages 555-556,describes the use of poly(vinyl alcohol) as a binder for ink-jetprinting paper. Poly(vinyl alcohol), 98-99% hydrolyzed and a 4%viscosity of 25-31 cP (Poval-PVA-117) was reported to be generally used.

An article in Tappi Journal, Vol.80, No.1, January 1997, pp. 68-70, byJohn Boylan, entitled, “Using Polyvinyl Alcohol in Ink-Jet PrintingPaper,” describes the use of various grades of poly(vinyl alcohol) forcoating paper. It is noted that partially hydrolyzed grades ofpoly(vinyl alcohol) provide the best printability in terms of inkoptical density and dry time when used with silica pigments in papercoatings. However, the final viscosity of poly(vinyl alcohol)/silicacoatings increases sharply with small increases in solids. Because ofthe viscosity increase, the maximum solids is about 25 to 30%, dependingon the grade of poly(vinyl alcohol). Partially hydrolyzed low/mediummolecular weight grades allow for the highest level of coating solids.

There are many patents on the use of poly(vinyl alcohol)as a pigmentbinder for paper coatings. For example:

U.S. Pat. No. 4,478,910 (Oshima et al., 1984) discloses ink jetrecording paper comprising a base sheet with a specific sizing degreehaving a coating layer comprising a water-soluble polymeric binder andfine silica particles. The silica particles have a specific surface areaof more than 200 m²/g and poly(vinyl alcohol) or its derivatives aredesired as binder because of their optical density. PVA 117,manufactured by Kuraray, was used in the examples.

U.S. Pat. No. 4,780,356 (Otouma et al., 1988) discloses a recordingsheet comprising a sheet of paper with porous particles on the papersurface. The porous particles (e.g., silica, silica-alumina, alumina,and silica-boria) have an average pore size of 10 to 5000 Å, a porevolume of 0.05 to 3.0 cc/g, and an average particle size of 0.1 to 50μm. Poly(vinyl alcohol) may be used as a binder for the particles in anamount of 5 to 60% (preferably 20 to 40%) by weight based on the totalweight of binder and particles. PVA 117, manufactured by Kuraray, wasused in the examples.

U.S. Pat. No. 5,057,570 (Miller et al., 1991) discloses a method ofpreparing a high solids, aqueous paper coating composition in which dryparticulate solids of a partially hydrolyzed, low molecular weightpoly(vinyl alcohol) is added to a high solids, aqueous pigmentdispersion and mixed, without external heating, until dissolved. Theaqueous pigment dispersion typically contains clay and/or calciumcarbonate at solids levels of 70 to 76%.

U.S. Pat. No. 5,270,103 (Oliver, 1993) discloses a receiver sheet havinga coating and suitable for printing with aqueous based inks, comprisinga pigment, poly(vinyl alcohol) binder, and an additional bindercomponent. The poly(vinyl alcohol) is at least 87 mole % hydrolyzed,preferably at least 99 mole % hydrolyzed.

JP 11-4983 (1999) discloses mixing poly(vinyl alcohol) with an organicand/or inorganic powder, and combining the mixture with water to obtaina non-lumping dispersion having a high concentration of poly(vinylalcohol). The dispersion is reported to be useful adhesives and paints.The poly(vinyl alcohol) powder has an average particle of 500 μm orless, a degree of polymerization of 500 to 3000 (preferably 100 to2500), and is 75 to 95 mole % (preferably 75 to 90 mole %) hydrolyzed.The two materials are blended in a volume ratio of 1/0.2 to 1/15poly(vinyl alcohol)/organic and/or inorganic particles. Examples ofinorganic particles are clays, silica, calcium carbonate, and bariumsulfate.

As noted above, fine particle size calcium carbonate has been shown tobe a particularly useful pigment in coating compositions for ink jetrecording paper; however the fine particle size results in a very highviscosity in the low shear rate range after the particles are put into aslurry at the levels needed for ink jet paper coating compositions. Thehigh viscosity in this low shear rate range presents problems inhandling the dispersion during the coating process.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed to producing a paper coatingcomposition having improved low shear viscosity at a high solids levelof fine particle size calcium carbonate. The improvement in low shearviscosity is achieved by dissolving, without heating and without addingwater, a fine particle size, partially hydrolyzed, low molecular weightpoly(vinyl alcohol) powder in an aqueous slurry of pigment particleswhich is predominantly fine particle size calcium carbonate. Thepoly(vinyl alcohol) has an average particle size of 200 μm or less, is85 to 90 mole % hydrolyzed, and has a degree of polymerization of 50 to600. The slurry, containing 0.1 to 50 parts poly(vinyl alcohol) per 100parts pigment particles, can then be formulated with other components toproduce a paper coating composition for specific applications such asink jet paper coatings.

There are several advantages to preparing a coating composition by firstmixing fine particle size, partially hydrolyzed, low molecular weightpoly(vinyl alcohol) powder directly to the fine particle size calciumcarbonate slurry. They include:

the poly(vinyl alcohol) does not need to be solubilized prior to mixingwith the calcium carbonate slurry, thus eliminating the problem ofadding more water to the slurry and reducing the amount of solids;

the poly(vinyl alcohol) can be solubilized in the calcium carbonateslurry without heating;

the low shear viscosity of the calcium carbonate slurry is significantlyreduced, thus allowing greater mixing efficiency, improvedfilterability, and improved pumping efficiency of the final coatingformulation;

the solids level of the pigment slurry can be increased withoutincreasing the shear viscosity, thus enabling easier handling of thefinal coating formulation;

binding of the calcium carbonate to a cellulosic substrate, despite itshigh surface area, is accomplished with a relatively small amount ofpoly(vinyl alcohol); e.g., as low as 5 to 15 parts of poly(vinylalcohol) per 100 parts pigment;

no additional binders are needed in the final coating formulation; and

the poly(vinyl alcohol)/calcium carbonate coating formulation, whenapplied to a paper substrate as an ink jet paper coating, providesexcellent ink jet printability.

DETAILED DESCRIPTION OF THE INVENTION

The aqueous pigment dispersion typically consists of at least about 90%by weight fine particle size calcium carbonate at solids levels rangingfrom 10 to 50%; preferably 20 to 30%. Up to about 10% of other paperpigments such as clays, silica, and titanium dioxide may also bepresent.

The fine particle size calcium carbonate has a mean surface area of atleast 50 m²/g; preferably at least 80 m²/g. Fine particle size calciumcarbonate can be prepared by heat aging and/or milling precipitatedcalcium carbonate, such as the method described in U.S. Pat. No.5,643,631 and U.S. Pat. No. 5,783,038. Calcium carbonate having a meansurface area of 80 m²/g is available commercially under the trademarkJETCOAT™ 30 Specialty PCC from Specialty Minerals.

Suitable fine particle size, low molecular weight, partially hydrolyzedpoly(vinyl alcohol) powder for use in this invention can be 70 to 90,preferably 85 to 90, and most preferably 87 to 89 mole % hydrolyzed,have a degree of polymerization (DPn) of 50 to 600, preferably 150 to300, and an average particle size of 200 μm or less; preferably, 180 μmor less. An example of a preferred poly(vinyl alcohol) powder is Airvol®203S poly(vinyl alcohol) supplied by Air Products and Chemicals, Inc.The poly(vinyl alcohol) used in this invention can be prepared bysynthesis and saponification techniques well-known to those skilled inthe art of manufacturing poly(vinyl alcohol). A fine particle size ofthe poly(vinyl alcohol) can be achieved by grinding the poly(vinylalcohol) particles and passing the particles through a mesh.

The fine particle size, low molecular weight, partially hydrolyzedpoly(vinyl alcohol) powder is slowly added to an agitated calciumcarbonate slurry at a rate that does not cause clumping of thepoly(vinyl alcohol). Typically, adding poly(vinyl alcohol) at a rate of1% of poly(vinyl alcohol) in 10 seconds is sufficient to preventclumping. Mixing is continued until the poly(vinyl alcohol) issolubilized; typically, mixing is continued at least 15 minutes. Mixingof the calcium carbonate slurry with the dry fine poly(vinyl alcohol)powder is preferably carried out at high shear rates. The amount ofpoly(vinyl alcohol) can range from 0.1 to 50 parts/100 parts of pigment;preferably 3 to 25 parts of poly(vinyl alcohol)/100 parts pigment.Amounts of 5 to 15 parts of poly(vinyl alcohol)/100 parts fine particlesize calcium carbonate have been found to efficiently bind the pigment.Solubilization of the poly(vinyl alcohol) can be carried out at ambienttemperature, i.e., 20° C. Heating is not required to solubilize thepoly(vinyl alcohol).

Low shear viscosity is the viscosity of a fluid (for example, calciumcarbonate slurry containing 28 to 32% solids and 3 to 25 parts lowmolecular weight, partially hydrolyzed poly(vinyl alcohol) per 100 partscalcium carbonate) which results from the shear rate generated by aBrookfield viscometer (No.3 spindle at 100 rpm).

The high solids aqueous pigment dispersion containing poly(vinylalcohol) can be used to prepare ink jet paper coating compositions orcan be used directly as an ink jet paper coating composition. Noadditional binders or dispersants are needed in the coating composition.A typical coating composition for ink jet paper applications contains:

90 to 100 parts fine particle size calcium carbonate;

0 to 10 parts secondary pigment;

0.1 to 50 parts poly(vinyl alcohol);

0 to 3 parts cationic dye fixatives such as polyethyleneimine orpoly(diallyldimethyl ammonium chloride); and

0 to 0.3 parts defoamer.

The invention will be further clarified by a consideration of thefollowing examples, which are intended to be purely exemplary of theinvention.

EXAMPLE 1 VISCOSITY MODIFICATION EFFECT

The viscosity modification effect of low molecular weight, partiallyhydrolyzed, fine particle size poly(vinyl alcohol) on slurries of fineparticle size calcium carbonate was measured. An aliquot of Jet Coat™ 30precipitated calcium carbonate (500g in a slurry containing 25 to 30%solids) was agitated with a high shear Dispersator type laboratorymixer. Various amounts of Airvol® 203S poly(vinyl alcohol) powder, inwhich 99% of the powder particles have an average particle size of lessthan 180 μm, were added to the agitated mixture at a rate of 1% per 10seconds. Agitation of the mixture was continued for 15 minutes afteraddition of the Airvol 203S. In a comparative example, 0.3 g oftetrasodium pyrophosphate (TSPP) dispersant was added to the calciumcarbonate slurry. The Brookfield Viscosity was measured at 10, 20, 50,and 100 rpm. Results of the measurements are shown in Table 1.

TABLE 1 5 pts 10 pts 20 pts 0.3 pt TSPP A 203S A 203S A 203S(dispersant) Jet per 100 per 100 per 100 per 100 Coat pts Jet pts Jetpts Jet pts Jet Coat 30* Coat 30 Coat 30 Coat 30 30 Solids 28.50% 28.04%29.72% 31.64% 27.70% Brookfield Viscosity No. 3 Spindle (cP) 10 rpm 69601530  990 900 5870 20 rpm 3980 810 545 550 3090 50 rpm 2008 358 264 3181308 100 rpm 1152 209 165 223  690 *Supplied by Specialty Minerals as aslurry.

With most coatings, as solids increase, low shear viscosity increases.However, unexpectedly, addition of the Airvol 203S to the Jet Coat 30resulted in a substantial reduction in low shear viscosity. Reduction oflow shear viscosity leveled out at 20 parts Airvol 203S/100 partscalcium carbonate. The results obtained with the Airvol 203S were muchbetter than those obtained with the TSPP dispersant.

EXAMPLE 2 BINDING EFFECT

The effect of Airvol 203S as a binder for fine calcium carbonateparticles on paper was measured. An uncoated base sheet of paper wassecured to a glass plate with tape. The coating formulation was pouredover the top width of the paper. A wire wound rod was placed at the topof the coating and drawn down the length of the paper applying a uniformapplication of the coating formulation across the length of the paper.The wet coated sheet was then dried in a forced air oven at 250° F. for2 minutes. The binding effect (IGT Pick Strength) was measured usingTappi Method T514 pm-82, “Surface Strength of Coated Paperboard.” Table2 presents the results of IGT pick strength measurements.

TABLE 2 0.3 pts TSPP 10 pts 20 pts (dispersant) Jet Coat 30 A 203S per100 A 203S per 100 per 100 pts (no A 203S) pts Jet Coat 30 pts Jet Coat30 Jet Coat 30 IGT Pick No bonding 9 14 No bonding Strength (WP)* *WP =viscosity velocity product (kilopoise-centimeters/second)

These data show that binding improved as the amount of Airvol 203Sincreased from 10 to 20 parts/100 parts Jet Coat 30. No bonding occurredusing Jet Coat 30 alone or Jet Coat 30 with TSPP.

EXAMPLE 3 PRINTABILITY

The ink jet printability of paper coated with a combination of Jet Coat30 and Airvol 203S poly(vinyl alcohol) was measured by applying thecoating formulation with a wire wound rod to an uncoated base sheet anddrying the coating at 250° F. for 2 minutes. Coat weights were between 8and 10 g/m². Airvol 203S alone and a mixture of 0.3 pt. TSPP with JetCoat 30 were used as comparative examples. The coated paper was printedon an Hewlett Packard HP 560 ink jet printer using a test patterndeveloped by Hewlett Packard. The optical density was measured with aTobias IQ 200 Densitometer. The results are presented in Table 3.

TABLE 3 10 pts 20 pts A 203S A 203S per 100 per 100 0.3 pts TSPP Basepts Jet pts Jet (dispersant) per Sheet A203S Coat 30 Coat 30 100 pts JetCoat 30 Comp 0.57 0.75 0.94 0.96 Black Magenta 0.77 N/A 1.14 1.1 Dusting Yellow 0.61 N/A 0.85 0.83 Cyan 1.03 1.11 1.48 1.4  Mono 0.860.94 1.32 1.24 Black

Ink jet printability of a binder used for ink jet paper coatings is veryimportant. The binder must be hydrophilic enough to allow the inkvehicle to penetrate into the coating while allowing the ink to remainat the surface of the coating with the pigment. In addition, the bindermust not contain undesirable surfactants which adversely effect thesurface energy of the coating causing the ink to spread creating highink dot gain leading to poor letter and image formation. One measure ofink jet printability is the ink optical density. The greater thedensity, the deeper the color shade produced.

The data in Table 3 show that the combinations of Jet Coat 30 and Airvol203S provide significantly better optical density of ink jet printingthan A203S alone. Airvol 203S is shown here to provide the hydrophilicproperty and it does not contain undesirable surfactants.

EXAMPLE 4 COMPARISON TO OTHER PIGMENTS

The example compares the effect of Airvol 203S on the low shearviscosity of several pigments typically used for paper coatingapplications. An aliquot of pigment slurry was weighed out and water wasadded, if necessary to obtain the desired solids level. Agitation wasthen begun with a laboratory type mixer. Airvol 203S was slowly added tothe agitating pigment slurry and mixing was continued for about 30minutes. Complete solubilization of the Airvol 203S was checked byrinsing a small sample through a 325 mesh screen and checking forunsolubilized poly(vinyl alcohol). When the poly(vinyl alcohol wascompletely dissolved, the final solids was measured via the microwavetechnique. The viscosity of the mixture was then measured with aBrookfield viscometer at 100 rpm. Results are presented in Table 4.

TABLE 4 Pigment Mean Airvol Surface 203S Solids (parts/ Brookfield RunArea % 100 parts Viscosity No. Pigment Type (m²/g) Solids pigment)(centipoise)  1 Calcium 12.6 70 0 85 Carbonate - A  2 Calcium 12.6 70 5775 Carbonate - A  3 Calcium 12.6 70 10 3252 Carbonate - A  4 Calcium12.6 70 20 4560 Carbonate - A  5 Calcium 12.6 30 0 15 Carbonate - A  6Calcium 12.6 30 5 19 Carbonate - A  7 Calcium 12.6 30 10 25 Carbonate -A  8 Calcium 12.6 30 20 40 Carbonate - A  9 Calcium 7.1 70 0 204Carbonate - B 10 Calcium 7.1 70 5 400 Carbonate - B 11 Calcium 7.1 70 102240 Carbonate - B 12 Calcium 7.1 70 20 2830 Carbonate - B 13 Calcium7.1 30 0 14.4 Carbonate - B 14 Calcium 7.1 30 5 18 Carbonate - B 15Calcium 7.1 30 10 25.1 Carbonate - B 16 Calcium 7.1 30 20 39 Carbonate -B 17 Jet Coat 30 80 30 0 1152 Calcium Carbonate 18 Jet Coat 30 80 30 5209 Calcium Carbonate 19 Jet Coat 30 80 30 10 165 Calcium Carbonate 20Jet Coat 30 80 30 20 223 Calcium Carbonate 21 Clay 15 70 0 241 22 Clay15 70 10 2140 23 Clay 15 30 0 17.2 24 Clay 15 30 10 31 25 TitaniumDioxide 7-30 70 0 125 26 Titanium Dioxide 7-30 70 10 288

The pigments of run no. 1-16 and 21-26 had low viscosities at 30% solidsdue to the greater particle size or reduced surface area of thesepigments compared to the Jet Coat 30 (run no. 17-20); i.e., the Jet Coat30 particles have a mean surface area which is 5 to 8 times the meansurface area of the other pigments in the example. It is well known thatas the surface area of pigment particles increase, the viscosity ofpigment slurries increase and addition of a binder, such as poly(vinylalcohol), will result in a further increase in viscosity. However,unexpectedly, when Airvol 203S was added to the Jet Coat 30 calciumcarbonate slurry (run no. 17-20), there was a substantial decrease inviscosity. In contrast, the viscosity increased when Airvol 203S wasadded to the other pigment slurry samples (run no. 1-16, 21-26).

What is claimed is:
 1. A method for preparing a paper coatingcomposition comprising an aqueous dispersion of fine particle sizecalcium carbonate, containing 20 to 30% solids, which comprises: mixingdry poly(vinyl alcohol) particles in an aqueous dispersion of calciumcarbonate particles, the calcium carbonate particles having a meansurface area of at least 60 m²/g, until the poly(vinyl alcohol)particles dissolve, to form a dispersion of calcium carbonate particlesin an aqueous poly(vinyl alcohol) solution, the dispersion of calciumcarbonate particles in the aqueous poly(vinyl alcohol) solutioncomprising 0.1 to 50 parts poly(vinyl alcohol) per 100 parts calciumcarbonate, the poly(vinyl alcohol) having a degree of polymerization of150 to 300 and being 87 to 89 mole % hydrolyzed, and the poly(vinylalcohol) particles having an average particle size of 180 μm or less. 2.The method of claim 1 wherein the mean surface area of the calciumcarbonate particles is 80 m²/g and the dispersion of calcium carbonateparticles in the aqueous poly(vinyl alcohol) solution comprises 3 to 25parts poly(vinyl alcohol) per 100 parts calcium carbonate.
 3. The methodof claim 1 wherein the dispersion of calcium carbonate particles in theaqueous poly(vinyl alcohol) solution comprises 5 to 15 parts poly(vinylalcohol)/100 parts calcium carbonate particles.
 4. The method of claim 1wherein the coating composition contains pigment particles consistingessentially of calcium carbonate particles having a mean surface area ofat least 60 m²/g.
 5. The method of claim 1 wherein the aqueousdispersion of pigment particles contains at least 90% by weight fineparticle size calcium carbonate having a mean surface area of at least60 m²/g.
 6. The method of claim 5 wherein the fine particle size calciumcarbonate has a mean surface area of at least 80 m²/g.